The present invention relates to holders, particularly collets or chucks, for holding high speed drills. When very fine drills are used, the rotational speeds have to become very high and indeed some systems use drills rotating at speeds in the region of 180,000 rpm. Such high speeds are necessary because as the diameter of the drill becomes less, its rotational speed has to increase so as to ensure a suitably high peripheral cutting speed. However with such very high rotational speeds, centrifugal forces become very large and they then act in opposition to forces holding the drill shank in position. This then creates inaccuracies in drilling performance.
The object of the present invention therefore is to provide a simple drill holder for high speed drilling and light routing which maintains a very high standard of accuracy, retains the drill bit tightly gripped, and is immune to the adverse affects of excessive speed, such as loosening of the drill due to centrifugal forces.
Accordingly the present invention provides a drill holder, for example a collet or chuck, for carrying a high speed drill, said drill holder comprising a collar arranged to enclose a drill shank, an outer rim support arranged for attachment to a drill, and a clamping mechanism between the collar and the outer rim support, said clamping mechanism being arranged to provide clamping forces between said outer rim support and said collar so that said collar tightly grips a drill shank, as a consequence of centrifugal forces and of off-centre sprag effect forces.
It should be explained that a sprag effect force is a force due to an offset pressure in a direction opposed to that of rotation. Sprag effect forces were originally used on railway waggons in the nineteenth century where a sprag, or piece of timber, was placed against a wagon wheel on a railway line at an orientation directed slightly above the centre line of the wheel to act as a brake. In such a case as the wheel tries to move towards the sprag the pressure increases due to the offset force of the sprag with respect to the axle of the wheel. Clutches have been devised based on the sprag effect where a wedging effect increases against the direction of rotation to provide for a clutching action.